Tomographic image capturing devices are put into practical use as a type of ophthalmic diagnostic device. The tomographic image capturing devices capture tomographic images of ocular fundi utilizing optical interference of so-called OCT (Optical Coherence Tomography).
There is a technique of arithmetically averaging a plurality of images to improve the image quality of an ocular fundus image obtained by the OCT. When the image processing of arithmetically averaging a plurality of images is performed, a speckle pattern disappears. The speckle pattern as referred to herein is an image pattern based on a phenomenon that portions of high intensity and low intensity of scattered light occur due to an indefinitely large number of superpositions of scattered light from a scattering body in the object to be measured. This is a physical phenomenon similar to a so-called interference phenomenon and the speckle pattern itself does not directly represent the structure of an ocular fundus that is the object to be measured. In this context, Patent Literature 1 discloses a technique of regarding the speckle pattern as noises and combining tomographic images thereby to cause the speckle pattern to disappear, thus obtaining a high-quality tomographic image.
On the other hand, Patent Literature 2 discloses an ultrasonograph that obtains an ultrasonographic image in a subject under test on the basis of echo signals of ultrasonic waves. This ultrasonograph is provided with an analysis algorithm capable of observing a lesion by positively taking advantage of a speckle pattern appearing in the ultrasonographic image and utilizing its statistical properties to smooth an image of the speckle part and extract a microstructure. This literature also discloses a technique of utilizing a phenomenon that the probability density distribution of brightness values of echo signals reflected from a normal liver follows the Rayleigh distribution and comparing the variance values to determine whether the probability density distribution obtained by actual measurement follows the Rayleigh distribution as the theoretical figure.
Non-Patent Literature 1 discloses that the speckle pattern of OCT can be quantified in accordance with the gamma distribution.